fix: bell badge sizing, text contrast, and empty timestamp display

- .tile-bell: inline-flex badge (min-width:16px height:16px) so it's a
  circle for single digits, pill for '9+'; was display:inline so
  width/height were ignored → oval shape
- .tile-bell color: #0D1117 (dark on amber) — was inheriting muted gray
  from .tile-meta, making count illegible
- formatTimestamp(null): return '' instead of em-dash '—' which rendered
  as a confusing glyph in the tile meta area

frontend/app.js

@@ -6,7 +6,7 @@
  * @returns {string}
  */
 function formatTimestamp(ts) {
-  if (ts == null) return '\u2014';
+  if (ts == null) return '';
   const diff = Math.floor(Date.now() / 1000 - ts);
   if (diff < 60) return `${diff}s ago`;
   if (diff < 3600) return `${Math.floor(diff / 60)}m ago`;